1. Field of the Invention
The present invention relates to a data processor system having look-ahead control. That is, the system is operated under an advanced control method by using prefetched instructions.
2. Description of the Related Art
The data processing capability, such as the processing speed, of a cental control unit (CC) used in a data processor system is determined by various factors. In a so-called Neumann-type data processor, in particular, the processing time of the data processor is largely accounted for by the access time necessary for accessing a main memory (MM) used in the data processor system.
In order to shorten the access time for high-speed processing, a variety of techniques have been proposed. One of these is a hierarchal memory. A typical hierarchal memory is a cache memory, which dramatically increases the operation speed of the data processor.
The cache memory method is, however, not preferable in view of software design freedom due to the variety of conditional restrictions necessary to be considered. More specifically, it is very important whether or not each desired or intended piece of data actually exists in the cache memory. This is usually defined as a "hit rate". Further, the cache memory must be fabricated as a high-speed memory, which necessarily increases the cost of the related data processor system.
In view of the above, it has long been hoped to realize a data processor system that is highly cost effective in performance, i.e., a low price data processor system however able to be operated with a very short instruction access time and high processing speed.
It is generally known to adopt the so-called bank division method in a data processor system for the purpose of speeding up the access to a main memory (MM) thereof. Further, it is generally known to adopt the so-called look-ahead method in a data processor system for the purpose of speeding up the process achieved by the central control unit (CC). The look-ahead method uses prefetched instructions by utilizing the fact that a series of instructions are usually stored sequentially one by one in the main memory (MM). A combination of the bank division method and the look-ahead method would be most preferable in increasing the operation speed of the overall data processing system.
In the prior art, there are two known method for performing look-ahead control, each of which methods will be explained in detail hereinafter. These two methods, however, each have their own disadvantages. The first method has the disadvantage that the higher the operation speed of the central control unit (CC), the lower the capability of look-ahead control. The second method has the disadvantage that the number of the interface lines in a memory bus between the main memory (MM) and the central control unit (CC) becomes very large.